Protection release method

ABSTRACT

In order to achieve improved release characteristics in the case of a protective device for power supply lines, a protection release method is proposed in which at least one signal (i, u) is supplied to a filter device (3, 16), and a release decision is made as a function of its output signal (SA, Z). If the conditions are insufficient for release, a change is made to the filter characteristics. The filter device (3, 16) in this case preferably comprises adaptive filters or combinations of filters. In a further refinement, a fuzzy region, which is used as the release criterion, is formed as a function of a change in the output signal.

BACKGROUND OF THE INVENTION

So-called protective devices which monitor lines for faults are used toprotect the operation of power supply lines. One such protective deviceis, for example, a distance protection device. Distance protectionmeasures the impedance of the line and monitors this line forundershooting an impedance value which can be predetermined.Undershooting occurs, for example, in the case of short circuits.

A problem with modern distance protective devices of digital design isthat filters are used which have a fixed window width and a fixedrelease criterion. In this case, there is a contradiction between thetwo criteria "high accuracy of impedance determination" and "shortcommand time for the off command". Specifically, if a protectionalgorithm is to operate quickly at a fixed technically sensible samplingfrequency, then digital filters having few coefficients must be used.When disturbance variables occur, such filters have large errors in theimpedance determination, which leads to a disturbance in theselectivity. In contrast, if a protection algorithm is to operate asselectively as possible, then filters having large numbers ofcoefficients must be used to damp interference variables. The protectionalgorithm then, however, has relatively long command times. This problemis known, for example, from "Proceedings of the 25th Universities PowerEngineering Conference", Aberdeen, UK, Sep. 12-14, 1990, pages 155 to158.

A test method is known from EP 0 284 546, in which filters having afixed window width are used.

SUMMARY OF THE INVENTION

The present invention is based on the object of specifying a signalmonitoring method in which the above-mentioned disadvantages of thecontradicting criteria are addressed.

Proceeding from the prior art, the inventor found a new way bydispensing with measured-value filters having fixed filtercharacteristics. Specifically, he confirmed that very inaccurate filtersare completely sufficient for certain faults, while high-precisionfilters are required only in some fault cases.

The object was achieved by a method including the steps of supplying atleast one signal to a filter device which has a time-variant filterbehavior deriving a fuzzy region from an output signal of said filterdevice, prompting a release when the fuzzy region is within a releaserange having limits which can be predetermined, and improving filtercharacteristics if the conditions are not sufficient for release.

A monitoring device includes a filter device for a signal, said devicehaving a time-variant filter behavior, and downstream-connected releaselogic in which a fuzzy region is derived from an output signal of thefilter device wherein a release signal is produced when the fuzzy regionis within a release range having limits which can be predetermined, andwherein a comparison is carried out with improved filter characteristicsif the conditions are not sufficient for release. In this way, a signalmonitoring method is available which is matched to the present faultsituation of the signal. In consequence, simple faults which requireonly a coarse filter function can be identified in a very short time andlead to a release, while faults which require a higher precision filterfunction are subject to more intensive filter processing. The methodachieves release times which are improved over the prior art in the vastmajority of cases.

A fuzzy region, which is used for the release decision, is preferablyderived from the output signal, the change with time of the outputsignal being used in particular. In consequence, a further reduction inthe release times can be achieved. In this case, the fuzzy region is tobe understood to mean the probability of how precisely the determinedimpedance (measured-value impedance) corresponds with the actual lineimpedance (measured impedance). The fuzzy region can be dependent on thefilter characteristic and/or the magnitude of a disturbance variable andis in this case used in the sense of a known measurement inaccuracy. If,for example, the output signal has a relatively large fuzzy region, theentire fuzzy region having to satisfy a release criterion, then apositive release decision can be made particularly quickly. In the caseof a negative release decision, the filter is improved until the fuzzyregion is constrained and a clear release decision can be made.

Fuzzy logic can be used in an advantageous manner for this function, itbeing possible to use weighing of disturbance variables in order to formthe fuzzy region. The improvement in the filter characteristic can inthis case be carried out continuously and automatically.

It is advantageous if a release range having limits which can bepredetermined is predetermined for the release decision. In consequence,the method can be matched to specific fault cases. In particular, themethod can be used for monitoring current, voltage, and, in particular,an impedance. The signals can in this case be formed as alternatingsignals. A preferred application is in the case of distance protection,an impedance signal then being monitored. The method has achieved verygood results in trials in this application, where it was not onlypossible to improve the release or command times of the distanceprotection, but also its precision.

Alternatively, fuzziness of the limits which can be predetermined canalso be formed. This option can possibly lead to simplification of themethod when designed as a program in a computer. The controllable filtercan also comprise a plurality of filter types, so that the most optimumfilter for the fault case is selected depending on the disturbancevariables identified. The filter can in this case also be designed to beauto-adaptive or auto-improving in the sense of a cyclic improvement. Inthis way, no feedback from release logic is necessary. The filter ispreferably designed to include an adaptive filter, especially an FIRfilter, with a number of support points which can be predetermined, andwherein filter characteristics are improved by increasing the number ofsupport points.

The design of the monitoring device is particularly simple if a digitalcomputer is used. The preferred application is given as a distanceprotective device for power supply lines. Further advantages of theinvention result from that which has been mentioned above, the otherclaims, and from the description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the following text, withreference to design variants by way of example.

FIG. 1 shows a block diagram of the monitoring device.

FIG. 2 shows a detailed block diagram.

FIGS. 3, 4 and 5 show examples of release polygons.

FIG. 6 shows a release diagram according to the prior art.

FIG. 7 shows a release diagram according to the present invention.

DETAILED DESCRIPTION

The block diagram according to FIG. 1 shows an arrangement, especially asignal monitoring device 1, in the case of which a signal S whichchanges with time is supplied to a filter device, especially a filter 3.The output signal SA is supplied to release logic 5, which makes arelease decision as a function of the output signal SA and, in the eventof a fault, produces and emits an off signal. The off signal can then besubjected to further processing or be used as an input signal for aswitching element, for example a power breaker.

The filter 3 is in this case designed as a variable filter, for exampleas an adaptive filter or as a combination of a plurality of filtertypes, which can be connected in parallel or in series. In this case,the filter 3 has an optimizing function, that is to say that the filterfunction is improved as a function of time. This can be done, forexample, by refining the filter 3 as an adaptive filter or by changingover between various filter types. This provides the option of an outputsignal SA which changes and qualitatively improves itself. Adaptivefilters are also known by the term FIR filters.

A release decision is made in the release logic 5 as a function of theoutput signal SA. A fuzzy region is initially formed for this purpose.The fuzzy region includes a statement on the probability of howprecisely the present output signal SA corresponds with the actual valueto be determined. In this case, a fuzzy region can be formed withrespect to the output signal SA or with respect to a predetermined limitof the release range of the release logic 5. As a result of this, if afault occurs in the signal S, a relatively large fuzzy region initiallyexists as a result of the filter 3 having a filter characteristic whichis admittedly not so good, but is fast. If the release criterion issatisfied despite the large amount of fuzziness, then an immediaterelease occurs. The probability that a fault is within the predeterminedlimits is satisfied.

If, however, an intersection occurs between the fuzzy region and therelease criteria, waiting takes place until an improvement of the filter3 allows an unambiguous decision. For this purpose, a control signal canalso be passed from the release logic 5, via a signal path 7, to thefilter 3 in order to improve the filter function. In this way, a controlloop is formed. The dashed line 9 indicates that the signal S can alsobe passed directly to the release logic 5 for identification ofdisturbance variable patterns (possibly with the interposition offurther processing).

FIG. 2 shows a more detailed illustration of the method as a blockdiagram, as it can preferably be used in the case of distance protectivedevices. For this purpose, a voltage signal u and a current signal i areinitially supplied to a measured-value filter 11. The measured-valuefilter 11 comprises a voltage filter 12, a first filter for the current,for example an exponential filter 13, and a further current filter 14.The current filter 14 and the voltage filter 12 are designed as variablefilters, especially as adaptive filters. The received signals U, I arethen supplied together with a frequency signal f to an impedance formingdevice 15. Its output signal is then supplied via the filter 3 to therelease logic 5. The arrangement of the modules 11, 15 and 3 in thiscase forms a filter device 16.

Fuzziness is formed in a detector 17. Direct detection of disturbancevariables can possibly also be carried out via additional signal lines,as indicated by the line 9, from the signals U, I. Furthermore,information from other protective devices can be supplied to thedetector 17 via a line 19 (comparative protection).

The release decision is made, and an off signal formed, in a releaseelement 21. In the event of impedance fuzziness, conformity with apredetermined limit polygon is monitored here. In the event of therelease conditions being insufficient, the variable filters 12, 14 and 3then receive a control signal via the signal paths 7a, 7b, which leadsto an improvement in the measurement precision for the impedance Z andto a reduction in the fuzziness. Fuzzy logic is preferably used in therelease logic 5 to form the fuzzy region and for release. The assessmentor weighing of the measured signal leads to a release decision which ison the one hand faster and is on the other hand more precise. Theconsequence of this is maximum selectivity of the protective device.More precise statements relating to specifically possible refinements ofthe individual modules of the protective device are made in thefollowing text.

With respect to the measured-value filter 11

The complex vectors are initially formed from the time profiles of thecurrent i and the voltage u, using modified linear-phased Fourieradaptive filters. This is done starting with a minimum number of supportpoints 3 in the current filter 14 and 7 in the voltage filter 12. Thenumber of support points is increased automatically. The linear-phasenature of the filters used has the advantage that any convolution makesdo with half the computation operations because of the symmetry in thefilter coefficients resulting therefrom. The offset in the supportpoints in the current path and voltage path can be used for the purposeof suppressing the aperiodic element in the short-circuit current withthe exponential filter 13. This element could otherwise be identified asa disturbance variable in the detector 17, which would lead to a delayin the off command.

When new sample values arrive after a fault has occurred or aftergeneral excitation, the level of the modified Fourier filters isincreased via the signal path 7a until all the disturbance variables aresufficiently damped and a reliable decision can be made in the releaseelement 21, which decision has the greatest probability of being thecorrect decision. A minor modification to known Fourier filters allows adamping change which is as large as possible to be achieved for thedisturbance variables during the design of the filters for the fuzzycontrol which is dependent on the disturbance variables.

With respect to the impedance forming device 15

The impedance forming device 15 is used to carry out the determinationof the resistance and of the reactance of the line as far as the faultpoint. The reactance can be corrected from the mains frequency to anominal frequency.

With respect to the filter 3

Investigations have shown that low-pass filtering of the impedances bymeans of square-wave or Hamming adaptive filters leads to the bestresults, which is justified by the fact that a stationary short circuitmust lead to a constant line reactance. The influence of moving shortcircuits is in consequence only somewhat smoothed.

With respect to the detector 17

The fuzziness is derived from the changes in the line reactance (and theresistance) during the filter adaptation in the measured-value filter 11and the filter 3 (fuzzy element). Measurement data from the oppositerelay, for example for improved fault localization, can also be takeninto account at this point in the control loop. This method step canpossibly also be carried out in the release element 21. In addition,disturbance variables can frequently be detected even in the inputsignals u and i or in the filtered signals U and I, using patternrecognition methods, which leads to premature adaptation of the filterfunctions to the disturbance variables.

With respect to the release element 21

The probability of the statement "final value of measured-valueimpedance is within the release polygon of the zone x" is essentiallydetermined here. If the probability is less than "1" (see also FIGS. 3,4 and 5), then the filters are adapted further in order to suppress thedisturbance variables better. The off command is not formed until theprobability is equal to "1". In the case of signals where thedisturbance variables are low, the probability becomes "1" even when thefilters have a relatively low window width. The off command can thus beformed very quickly and reliably. In the case of signals where thedisturbance variables are high and in the case of short circuits closeto the release polygon, the probability of 1 is not achieved until thewindow width is relatively large.

FIG. 3 shows a release polygon P which is formed from predeterminedlimits and on which an impedance Z is illustrated having a fuzzy regionU1. The fuzzy region U1 is not yet entirely located in the releasepolygon P. Release will thus not yet take place since the possibilityexists that the actual impedance as far as the short-circuit point isoutside the release polygon P.

FIG. 4 shows a condition which can occur after an improvement in thefilters--following the situation according to FIG. 3 or else at thestart of a measurement process. Despite a fuzzy region U2, a precisedecision on the fault case can be made since the entire fuzzy region U2is located within the release polygon P.

FIG. 5 shows an equivalent situation to this in which the fuzzy regionU3 is displaced into the release polygon P. The procedure is to be usedin the same sense as that described above.

FIG. 6 shows a command time response of a distance protective deviceaccording to the prior art in the event of a disturbance in themeasurement voltage as a result of different disturbance levels(harmonics). The various fault cases are shown. It can be seen that thedevice requires a minimum release time tmin. In the rear region (greaterthan 100%) of the curves shown, an inaccuracy in the distancedetermination also occurs, as well as non-selective release.

In the illustration according to FIG. 7, the release is shown accordingto the proposed method. The minimum release time tmin is considerablyreduced. In the rear region of the curve, a considerable improvement inthe distance determination can be found. Non-selective release isvirtually precluded. Small differences for different faults can still beidentified only in the central region. However, the results achieved arebetter than with the prior art in every case.

The proposed method and the associated device can, of course, also berefined within the context of the knowledge of a person skilled in theart. For example, general use for measured-valued processing or else forovercurrent time protection is conceivable. However, the preferredapplication is in the case of digital distance protection, in whichmicrocomputers and digital filters which are designed as a program areused. The renunciation of fixedly predetermined limits for a probabilityconsideration in order to form a release command leads to considerablyimproved protection characteristics.

What is claimed is:
 1. A protection release method, comprising thesteps:supplying at least one signal to a filter device which has atime-variant filter behavior; supplying an output signal of the filterdevice to a fuzzy logic device; deriving in the fuzzy logic device afuzzy region from the output signal of said filter device; prompting arelease when the fuzzy region is within a release range having limitswhich can be predetermined, and adjusting filter characteristics if theconditions are not sufficient for release.
 2. The protection releasemethod of claim 1, further comprising the step of employing a fuzzylogic to form said fuzzy region.
 3. The protection release method asclaimed in claim 1, wherein the fuzzy region is formed as either afunction of the filter characteristic or the magnitude of aninterference variable.
 4. The protection release method of claim 3,further comprising the step of employing a fuzzy logic to form saidfuzzy region.
 5. The protection release method of claim 1, wherein saidthe filter device has an adaptive filter, especially an FIR filter, witha number of support points which can be predetermined, and whereinfilter characteristics are improved by increasing the number of supportpoints.
 6. The protection release method of claim 5, further comprisingthe step of employing a fuzzy logic to form said fuzzy region.
 7. Theprotection release method of claim 1, wherein a change with time of theoutput signal is used to form said fuzzy region.
 8. The protectionrelease method as claimed in claim 7, wherein the fuzzy region is formedas either a function of the filter characteristic or the magnitude of aninterference variable.
 9. The protection release method of claim 7,wherein said the filter device has an adaptive filter, especially an FIRfilter, with a number of support points which can be predetermined, andwherein filter characteristics are improved by increasing the number ofsupport points.
 10. The protection release method of claim 7, furthercomprising the step of employing a fuzzy logic to form said fuzzyregion.
 11. A signal monitoring device comprising:a filter device for asignal, said device having a time-variant filter behavior, anddownstream-connected release fuzzy logic in which a fuzzy region isderived from an output signal of the filter device; wherein a releasesignal is produced when the fuzzy region is within a release rangehaving limits which can be predetermined; and wherein a comparison iscarried out with modified filter characteristics if the conditions arenot sufficient for release.
 12. The signal monitoring device of claim11, wherein the fuzzy region is derived in the release logic from thechange with time of the output signal.
 13. The signal monitoring deviceof claim 12, wherein said filter device has at least one adaptivefilter, especially an FIR filter, with a number of support points whichcan be predetermined, and the filter characteristics are improved byincreasing the number of support points.
 14. The signal monitoringdevice of claim 12, wherein a changeover is made from a first filtertype to a second filter type to improve the filter characteristic. 15.The signal monitoring device of claim 12, further comprising a fuzzyelement for forming the fuzzy region.
 16. The signal monitoring deviceof claim 12, wherein the release logic has an interface for datainterchange with further devices.
 17. The signal monitoring device ofclaim 11, wherein said filter device has at least one adaptive filter,especially an FIR filter, with a number of support points which can bepredetermined, and the filter characteristics are improved by increasingthe number of support points.
 18. The signal monitoring device of claim17, wherein a changeover is made from a first filter type to a secondfilter type to improve the filter characteristic.
 19. The signalmonitoring device of claim 17, further comprising a fuzzy element forforming the fuzzy region.
 20. The signal monitoring device of claim 17,wherein the release logic has an interface for data interchange withfurther devices.
 21. The signal monitoring device of claim 17, whereinthe release logic has an interface for data interchange with furtherdevices.